<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
schema	O
versioning	O
and	O
schema	B-Application
evolution	I-Application
,	O
deal	O
with	O
the	O
need	O
to	O
retain	O
current	O
data	O
and	O
software	O
system	O
functionality	O
in	O
the	O
face	O
of	O
changing	O
database	O
structure	O
.	O
</s>
<s>
A	O
database	O
design	O
is	O
sometimes	O
created	O
as	O
a	O
"	O
as	O
of	O
now	O
"	O
instance	O
and	O
thus	O
schema	B-Application
evolution	I-Application
is	O
not	O
considered	O
.	O
</s>
<s>
This	O
assumption	O
,	O
almost	O
unrealistic	O
in	O
the	O
context	O
of	O
traditional	O
information	O
systems	O
,	O
becomes	O
unacceptable	O
in	O
the	O
context	O
of	O
systems	O
that	O
retain	O
large	O
volumes	O
of	O
historical	O
information	O
or	O
those	O
such	O
as	O
Web	B-Application
Information	I-Application
Systems	I-Application
,	O
that	O
due	O
to	O
the	O
distributed	B-Architecture
and	O
cooperative	O
nature	O
of	O
their	O
development	O
,	O
are	O
subject	O
of	O
an	O
even	O
stronger	O
pressure	O
toward	O
change	O
(	O
from	O
39%	O
to	O
over	O
500%	O
more	O
intense	O
than	O
in	O
traditional	O
settings	O
)	O
.	O
</s>
<s>
Due	O
to	O
this	O
historical	O
heritage	O
the	O
process	O
of	O
schema	B-Application
evolution	I-Application
is	O
nowadays	O
a	O
particularly	O
taxing	O
one	O
.	O
</s>
<s>
of	O
the	O
schema	B-Application
evolution	I-Application
on	O
queries	O
and	O
applications	O
.	O
</s>
<s>
Supporting	O
Schema	B-Application
Evolution	I-Application
is	O
a	O
difficult	O
problem	O
involving	O
complex	O
mapping	O
among	O
schema	O
versions	O
and	O
the	O
tool	O
support	O
has	O
been	O
so	O
far	O
very	O
limited	O
.	O
</s>
<s>
The	O
recent	O
theoretical	O
advances	O
on	O
mapping	O
composition	O
and	O
mapping	O
invertibility	O
,	O
which	O
represent	O
the	O
core	O
problems	O
underlying	O
the	O
schema	B-Application
evolution	I-Application
remains	O
almost	O
inaccessible	O
to	O
the	O
large	O
public	O
.	O
</s>
<s>
A	O
rich	O
bibliography	O
on	O
Schema	B-Application
Evolution	I-Application
is	O
collected	O
at	O
:	O
</s>
<s>
UCLA	O
university	O
carried	O
out	O
an	O
analysis	O
of	O
the	O
MediaWiki	O
Schema	B-Application
Evolution	I-Application
:	O
</s>
<s>
PRISM	O
,	O
a	O
tool	O
to	O
support	O
graceful	O
relational	O
schema	B-Application
evolution	I-Application
:	O
</s>
<s>
Pario	O
and	O
deltasql	O
are	O
examples	O
of	O
software	O
development	O
tools	O
that	O
include	O
fully	O
automated	O
schema	B-Application
evolution	I-Application
.	O
</s>
